A major factor associated with both acute and chronic heart problems is the reduction in blood flow to the heart muscle resulting from restrictions in the coronary blood vessels. In many cases such restrictions are caused by deposits of atherosclerotic plaque deposited on the walls of blood vessels which cause an abnormal narrowing of the lumen or blood vessel channel. When the lumen is sufficiently narrowed that the rate of blood flow restriction becomes critical a variety of medical procedures and techniques are available to the intervening cardiologist in order to restore adequate blood flow. The most common of the currently available medical procedures for treating such stenotic lesions (the abnormal narrowing of a blood vessel due to injury or vascular disease) include pharmacological treatments which dilate the blood vessels, bypass surgery to shunt blood around the lesions, and balloon angioplasty to reopen the narrowed vessels.
Each medical procedure has its own benefits and drawbacks. However, over the last decade balloon angioplasty has become widely accepted as a safe and effective method for treating such vascular diseases in appropriate circumstances. The most common form of angioplasty is called percutaneous transluminal coronary angioplasty or PTCA. Generally, this procedure utilizes a dilatation catheter provided with an inflatable balloon at its distal end. Using a fluoroscope and radiopaque dyes for visualization the interventional cardiologist guides the distal end of the dilation catheter through the vascular lumen into a position across the stenotic lesion. Once positioned the balloon is inflated for a brief duration to displace or otherwise reopen the lesion and reestablish adequate blood flow.
Typically, inflation of the balloon is accomplished by supplying a pressurized fluid through an inflation lumen in the catheter which is connected to an inflation apparatus located outside of the patient's body. Similarly, applying a negative pressure to the inflation lumen collapses the balloon to its minimum dimension for initial placement of the balloon catheter within or removal from the target blood vessel.
A wide variety of angioplasty balloon catheter designs and constructions are available to the vascular physician. Typically, the catheter is constructed of resilient or elastomeric materials configured to produce a specific balloon inflation diameter at a standard inflation pressure of six atmospheres. Because vascular lesions differ in size most balloon catheters are available in stepped dilation diameters ranging from approximately 1.5 millimeter to 4.0 millimeter in 0.5 millimeter or 0.25 millimeter increments. Accordingly, after locating the stenosis in an artery or vessel utilizing a procedure such as an angiogram, a cardiologist is able to guage the size of the lesion and select the appropriately sized balloon to effectively open the lesion.
However, in spite of the high level of training and skill exhibited by today's cardiologists, selecting the appropriately sized balloon catheter is complicated by a variety of factors. In many cases, stenotic lesions exhibit a markedly irregular cross sectional configuration which, when viewed from differing angles, may present deceptively narrow profiles. Additionally, the composition of the stenotic lesion itself may vary from hard, calcified materials to soft, readily displaceable deposits. Because utilization of an inappropriately large balloon may result in undesirable damage to the vascular tissue surrounding the lesion, out of an abundance of caution, balloon size selection generally is biased toward the smallest inflation diameter that will effectively open the stenosis without inducing trauma to the vascular tissue. As a result, when unexpectedly soft or irregularly shaped lesions are encountered what would appear to have been an appropriately sized balloon at first may in fact be undersized and less than effective.
In order to address these problems without the need to replace the initially sized balloon catheter with a larger sized apparatus, many of the current generation of dilation balloons are constructed with a predetermined overinflation diameter which may be as much 20% greater than the standard reference inflation diameter. As a result, the intervening cardiologist may reinflate the balloon catheter to a higher pressure in order to achieve a slightly greater dilation diameter that may effectively open the lesion.
During such initial inflation or reinflation it is desirable for the physician to know the pressure to which the balloon is being inflated in order to insure that the proper diameter has been achieved. At present, inflation pressure usually is displayed by an analog pressure gauge or digital readout which enables the cardiologist to determine that the balloon catheter is being used within its operational pressure range (typically 6 to 12 atmospheres) and below its burst pressure (typically ranging from 10 to 20 atmospheres depending upon balloon construction and materials). Though the current generation of analog pressure gauges are reasonably accurate and easy to read, their accuracy is no better than one half the smallest increment (typically 1/2 to 1/4 atmospheres). More importantly, at present there is no convenient way for the cardiologist to determine balloon inflation diameter with any degree of accuracy beyond that expected at the reference inflation pressure of 6 atmospheres. Current technology requires the cardiologist to refer to a graph of balloon inflation diameter versus pressure which is routinely supplied by the catheter manufacturer. Visual interpolation of this graph during angioplasty is difficult at best due to the darkened lighting typically present in the operating room (to assist flouroscopic visualization) and the demanding aspects of the procedure itself.
Accordingly, there is a need for an apparatus and method that will effectively communicate real time inflation balloon diameters and pressures to a vascular physician during a balloon dilation procedure. Additionally, there is a need to enable a vascular physician to accurately manipulate the effective diameter of an inflation balloon over as wide a range of dimensions and pressures as possible while remaining below the balloon burst pressure. Such an apparatus should be simple and easy to use, relatively inexpensive, and capable of interfacing with existing operating room monitors and displays if desired.